The present invention relates to an ink jet printing head and an ink jet printing device for printing letters, images, etc. on a medium such as paper by ejecting ink drops from nozzles, and in particular, to an ink jet printing head and an ink jet printing device capable of realizing high-frequency ink drop ejection and high-speed printing.
In a drop-on-demand ink jet printer, a pressure wave (acoustic wave) is generated in a pressure generation chamber which is filled with ink by use of pressure generation means such as a piezoelectric actuator and thereby an ink drop is ejected from a nozzle that is connected with the pressure generation chamber. Such drop-on-demand ink jet printers are well known today as disclosed in Japanese Publication of Examined Patent Applications No.SH053-12138, Japanese Patent Application Laid-Open No.HEI10-193587, etc.
FIG. 1 is a cross-sectional view showing an example of a printing head of a conventional ink jet printing head. Each pressure generation chamber 121 of the printing head is connected with a nozzle 122 for ejects ink drops and an ink supply channel 124 for guiding ink from an ink tank (unshown) to the pressure generation chamber 121 via a common ink channel 123.
To the bottom of the pressure generation chamber 121, a vibration plate 125 is provided. In order to eject an ink drop, a piezoelectric actuator 126 which is provided outside the pressure generation chamber 121 deforms the vibration plate 125 and thereby changes the volume (capacity) of the pressure generation chamber 121. The change of volume causes a pressure wave in the pressure generation chamber 121 and thereby part of the ink packed in the pressure generation chamber 121 is ejected from the nozzle 122 to outside as an ink drop 127. The ink drop 127 flying from the nozzle 122 reaches a medium such as paper and thereby forms an ink dot. Letters, images, etc. are printed and recorded on the medium by repeating the ink dot formation according to specific image data.
Various types of driving waveforms are applied to the piezoelectric actuator 126 depending on the size of the ink drop 127 to be ejected from the nozzle 122. For the ejection of large-diameter ink drops 127 for printing letters, deep-color parts, etc., a driving waveform as shown in FIG. 2 is generally used. First, the voltage applied to the piezoelectric actuator 126 is raised (voltage increase process 111), thereby the volume of the pressure generation chamber 121 is rapidly decreased and thereby the ink drop ejection is carried out. Thereafter, the voltage is returned to the bias voltage Vb (voltage decrease process 112).
FIG. 3 is a schematic diagram showing the action of a meniscus in a nozzle when the ink drop ejection is carried out. The meniscus 132 which is almost flat in the beginning ((A) of FIG. 3) moves outward as the pressure generation chamber 121 is compressed, and thereby an ink drop 133 is ejected from the nozzle 131 ((B) of FIG. 3). Due to the decrease of ink in the nozzle 131 caused by the ink drop ejection, a concave meniscus 132 is formed in the nozzle 131 ((C) of FIG. 3). The surface of the concave meniscus 132 gradually returns to the nozzle opening due to surface tension of ink and thereafter recovers to the original state, that is, the state before the ink drop ejection ((D), (E) and (F) of FIG. 3).
FIG. 4 is a graph showing the change of position of the meniscus when the ink drop ejection is carried out. As shown in FIG. 4, the meniscus 132 which withdrew widely (y=xe2x88x9260 xcexcm) just after the ink drop ejection (t=0) returns to the initial position (y=0) after vibrating. In this document, the action of the meniscus returning to the initial position after the ink drop ejection will be referred to as xe2x80x9crefillxe2x80x9d, and the time (tr) necessary for the meniscus to return first to the nozzle opening surface (y=0) after the ink drop ejection will hereafter be referred to as xe2x80x9crefill timexe2x80x9d.
When the repeated and continuous ink drop ejection is carried out by an ink jet printing head, if an ink drop is ejected before the refill after the previous ink drop ejection is completed, the uniformity of the diameter and speed of ink drops is deteriorated and thereby the continuous ink drop ejection becomes unstable. In other words, stable ink drop ejection is impossible until a time tr or more elapses after the previous ink drop ejection. Therefore, the refill time tr is a critical characteristic value dominating the maximum ejection frequency (printing speed) of an ink jet printing head.
Besides the refill time tr, the number of nozzles also dominates the printing speed. As the number of nozzles increases, the number of dots that can be formed in a unit time increases and thereby the printing speed increases. Therefore, in ordinary ink jet printers of these days, a multi-nozzle printing head, having a plurality of ink drop ejection mechanisms (ejectors) which are connected together, is generally employed.
FIG. 5 is a schematic diagram showing the basic composition of a multi-nozzle ink jet printing head. An ink tank 157 is connected with a common ink channel 153. To the common ink channel 153, a plurality of pressure generation chambers 151 are connected via ink supply channels (unshown). By such composition, the ink drop ejection can be carried out from a plurality of ejectors at the same time and thereby the printing speed can be increased.
However, in order to realize stable ink drop ejection in such a multi-nozzle ink jet printing head, the common ink channel has to be designed properly, that is, pressure wave interference (crosstalk) etc. between the ejectors (which are connected with the common ink channel) has to be eliminated. Therefore, some methods for preventing the crosstalk between the ejectors by enlarging the acoustic capacitance of the common ink channel have been proposed so far.
For example, in an ink jet printing head disclosed in Japanese Patent Application Laid-Open No.SHO56-75863 (hereafter, referred to as xe2x80x9cprior art #1xe2x80x9d), the capacity of the common ink channel is set to more than twice as large as the total capacity of the pressure generation chambers (including nearby channels) and thereby the crosstalk is suppressed.
In Japanese Patent Application Laid-Open No.SHO52-49034 and Japanese Patent Application Laid-Open No.HEI9-141864, pressure damping means (air damper, pressure absorber, etc.) is provided to the common ink channel in order to realize a large acoustic capacitance even in a common ink channel of a limited capacity.
In Japanese Patent Application Laid-Open No.SHO59-26269 (hereafter, referred to as xe2x80x9cprior art #2xe2x80x9d), based on the number (N) of ejectors connected with the common ink channel and the impedance (ZS) of the ink supply channel, the impedance (ZR) of the common ink channel is set so as to satisfy a condition ZRxe2x89xa6ZS/(10N) and thereby the crosstalk is suppressed.
However, according to evaluations of experimentally manufactured multi-nozzle ink jet printing heads which have been performed by the present inventors, it became clear that the conventional ink jet printing heads explained above are not necessarily capable of guaranteeing stable ink drop ejection. The problems with the conventional ink jet printing heads will hereafter be explained referring to some concrete examples.
First, when a plurality of ejectors (which are connected together by the common ink channel) carries out the ink drop ejection simultaneously, the refill time of each ejector increases, and further, variation of refill time occurs between ejectors.
FIG. 6 is a graph showing an experimental result of the refill time of the conventional multi-nozzle ink jet printing head of FIG. 5. In the experiment, a multi-nozzle ink jet printing head having 32 ejectors was used, and the refill time of each ejector was measured under different ejection conditions. An air damper was provided to the common ink channel and thereby the acoustic capacitance of the common ink channel was set large enough to satisfy the conditions of the prior arts #1 and #2. In FIG. 5, the ejectors are numbered from #1 (ejector nearest to the inlet (joint between the ink tube B 155 and the common ink channel 153) of the common ink channel) to #32 (ejector farthest from the inlet).
When each ejector was driven individually and separately, the refill time was almost constant (approximately 50 xcexcs) for all the ejectors as shown by open circles (◯) of FIG. 6. On the other hand, when all the 32 ejectors were driven at the same time and the simultaneous ink drop ejection was carried out, the refill time increased as a whole, and the variation of the refill time between ejectors occurred considerably. Concretely, the increase of refill time was relatively small and almost constant in the ejectors #1xcx9c#25, whereas the refill time exhibited a rapid increase from the ejector #26. The refill time increased to 65 xcexcs in the ejector #32 at the distal end of the common ink channel.
As above, in the conventional multi-nozzle ink jet printing heads, the refill time tends to increase much in ejectors near the distal end of the common ink channel when the simultaneous ink drop ejection from all the ejectors is carried out. Such phenomenon becomes prominent as the number of ejectors connected to the common ink channel becomes larger.
If such variation of refill time occurs, the maximum ejection frequency of the head is necessitated to be decreased and thereby high-speed printing becomes difficult. In the above example, each ejector should be capable of ejecting ink drops at a frequency of approximately 20 kHz since the refill time of each ejector is 50 xcexcs in the single (separate) ink drop ejection. However, when the simultaneous ink drop ejection (from all the ejectors) is carried out, the refill time of the ejector #32 increases to 65 xcexcs, thereby the maximum ejection frequency in practical use drops to 15 kHz. If the ink drop ejection at 20 kHz is forcibly carried out, the ink drop ejection state becomes very unstable and at worst, the ink drop ejection capabilities of the nozzles are disabled by bubbles taken in the nozzles. Even when the ejection frequency was lowered to 15 kHz, variations of xc2x115% in the ink drop volume and xc2x118% in the ink drop speed were observed between the ejectors.
Such variations in the ink drop volume and ink drop speed can be attributed to unevenness of the meniscus initial state ((A) of FIG. 3) between the ejectors which is caused by the refill speed variation between the ejectors. Incidentally, when the simultaneous ink drop ejection from all the nozzles was carried out at a far lower ejection frequency (1 kHz), the variations in the ink drop volume and ink drop speed were both within xc2x12%, that is, the pressure wave interference (crosstalk) between ejectors was almost perfectly eliminated at the low frequency.
As explained above, in the conventional ink jet printing heads, the refill time increases and the refill time variation between nozzles occurs when the simultaneous ink drop ejection from all the nozzles is carried out, thereby stable ink drop ejection from the nozzles at high frequency becomes difficult. The phenomenon puts limitations both on the number of ejectors and on the ejection frequency, seriously obstructing the improvement of the printing speed of ink jet printers.
Further, as the second problem of the conventional ink jet printing heads, the crosstalk can not necessarily be eliminated even if the common ink channel characteristics (acoustic capacitance, impedance) are adjusted according to conventional techniques. The crosstalk sometimes cause large variations in the ink drop volume and ink drop speed.
FIG. 7A is a graph showing the change of the incidence of crosstalk that was observed by the present inventors when the ratio between the capacity Wp of the common ink channel and the total capacity Wc of the pressure generation chambers (including nearby channels) was changed in the conventional multi-nozzle ink jet printing head of FIG. 5 (having 32 ejectors and no air damper). The incidence of crosstalk was obtained from the variation occurring in the ink drop speed. As is clear from FIG. 7A, crosstalk diminishes as the ratio Wp/Wc increases.
However, FIG. 7A also shows that the crosstalk can not be suppressed perfectly even when the ratio Wp/Wc is set larger than 2 according to the prior art #1. Especially in the range 0.1 less than Wp/Wc 10, the crosstalk becomes much dominant and the variations in the ink drop volume and ink drop speed amount to 30% or more.
FIG. 7B is a graph showing the change of the ratio (ZS/(ZRxc2x7N)) between supply channel impedance ZS and common ink channel impedance ZR that was observed by the present inventors when the ratio Wp/Wc was changed in the ink jet printing head of FIG. 5. In the evaluated head, the ratio ZS/(ZRxc2x7N) becomes 10 or more when Wp/Wc greater than 6, by which the condition (ZRxe2x89xa6ZS/(10N)) of the prior art #2 is satisfied.
However, crosstalk occurs when Wp/Wc less than 50 as shown in FIG. 7A, which means that the condition of the prior art #2 is also not a sufficient condition for preventing the crosstalk. The prior art #2 sets the common ink channel impedance ZR based on the supply channel impedance ZS only, without taking other factors (acoustic capacitance of the pressure generation chambers, etc.) into consideration.
As explained above, the conventional ink jet printing heads have the second problem of being incapable of necessarily eliminating the crosstalk between the ejectors even if the characteristics and structure of the common ink channel are set properly according to the conventional techniques.
As described above, in the conventional ink jet printing heads, the refill time increases and the refill time variation between nozzles occurs when the simultaneous ink drop ejection from nozzles is carried out (first problem), and the crosstalk can not be eliminated perfectly (second problem). The problems become critical as the number of ejectors connected to the common ink channel gets larger, by which the realization of high-speed ink jet printing heads becomes difficult.
It is therefore the primary object of the present invention to provide an ink jet printing head and an ink jet printing device by which the crosstalk and the increase of the refill time occurring in the simultaneous ink drop ejection from many ejectors can be avoided and thereby high-speed and high-quality printing can be realized.
In accordance with a first aspect of the present invention, there is provided an ink jet printing head comprising: a plurality of ejectors (each of which at least includes a pressure generation chamber, a nozzle which is connected with the pressure generation chamber, and pressure generation means for generating pressure in the pressure generation chamber); and an ink supply system (which at least includes a common ink channel to which the ejectors are connected), and forming letters, image patterns, etc. on a medium by ejecting ink drops from the nozzles by letting the pressure generation means cause change of pressure in the pressure generation chambers which are filled with ink supplied from the common ink channel. In the ink jet printing head, acoustic capacitance Cp of the common ink channel per ejector is set based on acoustic capacitance Cn of the nozzle and acoustic capacitance Cc of the pressure generation chamber.
In accordance with a second aspect of the present invention, in the first aspect, the acoustic capacitance Cp of the common ink channel per ejector is set so as to satisfy a condition Cp greater than 10Cn.
In accordance with a third aspect of the present invention, in the first aspect, the acoustic capacitance Cp of the common ink channel per ejector is set so as to satisfy a condition Cp greater than 20Cc.
In accordance with a fourth aspect of the present invention, in the first aspect, the acoustic capacitance Cp of the common ink channel per ejector is set so as to satisfy conditions Cp greater than 10Cn and Cp greater than 20Cc.
In accordance with a fifth aspect of the present invention, in the first aspect, the common ink channel is provided with pressure damping means for damping pressure therein.
In accordance with a sixth aspect of the present invention, in the fifth aspect, the pressure damping means is implemented by a resin film.
In accordance with a seventh aspect of the present invention, in the sixth aspect, the pressure damping means is implemented by a polyimide film.
In accordance with an eighth aspect of the present invention, in the fifth aspect, the pressure damping means is formed so that its acoustic capacitance will get larger at the distal end of the common ink channel.
In accordance with a ninth aspect of the present invention, in the eighth aspect, the thickness of the pressure damping means is decreased at the distal end of the common ink channel.
In accordance with a tenth aspect of the present invention, in the eighth aspect, grooves are provided to the pressure damping means at the distal end of the common ink channel.
In accordance with an eleventh aspect of the present invention, in the first aspect, an area to which no ejector is connected is provided to the distal end of the common ink channel.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, a hole or channel for bleeding bubbles is provided to the distal end of the common ink channel.
In accordance with a thirteenth aspect of the present invention, in the first aspect, the ejectors are arranged in a two-dimensional matrix.
In accordance with a fourteenth aspect of the present invention, in the thirteenth aspect, the common ink channel includes: a common ink channel mainstream as the upstream side of the common ink channel; and a plurality of common ink channel tributaries which are connected to the common ink channel mainstream as the downstream side of the common ink channel, to each of which a plurality of ejectors are provided.
In accordance with a fifteenth aspect of the present invention, there is provided an ink jet printing device employing an ink jet printing head that comprises: a plurality of ejectors (each of which at least includes a pressure generation chamber, a nozzle which is connected with the pressure generation chamber, and pressure generation means for generating pressure in the pressure generation chamber); and an ink supply system (which at least includes a common ink channel to which the ejectors are connected), and that forms letters, image patterns, etc. on a medium by ejecting ink drops from the nozzles by letting the pressure generation means cause change of pressure in the pressure generation chambers which are filled with ink supplied from the common ink channel. In the ink jet printing head of the ink jet printing device, acoustic capacitance Cp of the common ink channel per ejector is set based on acoustic capacitance Cn of the nozzle and acoustic capacitance Cc of the pressure generation chamber.
In accordance with a sixteenth aspect of the present invention, in the fifteenth aspect, the acoustic capacitance Cp of the common ink channel per ejector is set so as to satisfy a condition Cp greater than 10Cn.
In accordance with a seventeenth aspect of the present invention, in the fifteenth aspect, the acoustic capacitance Cp of the common ink channel per ejector is set so as to satisfy a condition Cp greater than 20Cc.
In accordance with an eighteenth aspect of the present invention, in the fifteenth aspect, the acoustic capacitance Cp of the common ink channel per ejector is set so as to satisfy conditions Cp greater than 10Cn and Cp greater than 20Cc.
In accordance with a nineteenth aspect of the present invention, in the fifteenth aspect, the common ink channel is provided with pressure damping means for damping pressure therein.
In accordance with a twentieth aspect of the present invention, in the nineteenth aspect, the pressure damping means is implemented by a resin film.
In accordance with a twenty-first aspect of the present invention, in the twentieth aspect, the pressure damping means is implemented by a polyimide film.
In accordance with a twenty-second aspect of the present invention, in the nineteenth aspect, the pressure damping means is formed so that its acoustic capacitance will get larger at the distal end of the common ink channel.
In accordance with a twenty-third aspect of the present invention, in the twenty-second aspect, the thickness of the pressure damping means is decreased at the distal end of the common ink channel.
In accordance with a twenty-fourth aspect of the present invention, in the twenty-second aspect, grooves are provided to the pressure damping means at the distal end of the common ink channel.
In accordance with a twenty-fifth aspect of the present invention, in the fifteenth aspect, an area to which no ejector is connected is provided to the distal end of the common ink channel.
In accordance with a twenty-sixth aspect of the present invention, in the twenty-fifth aspect, a hole or channel for bleeding bubbles is provided to the distal end of the common ink channel.
In accordance with a twenty-seventh aspect of the present invention, in the fifteenth aspect, the ejectors are arranged in a two-dimensional matrix.
In accordance with a twenty-eighth aspect of the present invention, in the twenty-seventh aspect, the common ink channel includes: a common ink channel mainstream as the upstream side of the common ink channel; and a plurality of common ink channel tributaries which are connected to the common ink channel mainstream as the downstream side of the common ink channel, to each of which a plurality of ejectors are provided.